The use of sonic and ultrasonic compressional waves to pasteurize liquids is well known in the art. Before considering the limitations and variations of prior art pasteurization methods, it is first advisable to consider the most widely recognized mechanism through which compressional waves destroy bacteria an other microorganisms.
U.S. Pat. No. 4,211,744 to Boucher explains that cavitation is the mechanism through which microorganisms are killed. Cavitation of a liquid using compressional waves involves the creation of small bubbles of vapor formed from the liquid involved. These small bubbles are formed in the decompression region behind the compressional wave front. As the magnitude of the pressure fluctuations in compressional waves increase, there is greater tendency for the liquid to vaporize into small bubbles. When the pressure front strikes these vapor bubbles, they implode. The implosion is believed to tear away small portions of the microorganisms or to physically break them into parts. Repeated cavitational attack tends to disintegrate the bacteria or other microorganisms thereby pasteurizing the liquid involved.
Prior art compressional wave pasteurization methods have used both sonic and ultrasonic frequencies. U.S. Pat. No. 2,138,839 to Chambers discloses a method using sonic compressional waves and static pressures of about 60 pounds per square inch or greater. Although Chambers recognizes the bacteriacidal action of cavitation, he explains that in his invention cavitation is not necessary to kill bacteria because of the application of relatively high static pressure. The high static pressure also prevents milk from homogenizing and keeps dissolved gases in beer.
U.S. Pat. No. 2,138,052 to Williams uses sonic frequency waves having very high acoustic pressure amplitudes, on the order of 10,000 atmosphers, to kill bacteria. The use of cavitation is avoided for the same reasons as explained above with regard to Chambers.
The invention shown in U.S. Pat. No. 2,417,722 to Wolff purifies liquids by subjecting them to either sonic or ultrasonic frequency compressional waves at acoustic pressure amplitudes of 100 to 200 bars. Wolff also bubbles oxygen gas through the liquid while subjecting it to these high pressure compressional waves. Wolff explains that the oxygen forms ozone which greatly accelerates the purification process.
U.S. Pat. No. 3,672,823 to Boucher explains that 100% kill rates are very difficult to obtain even using exposure times of 30 minutes or greater with acoustic power densities of several watts per cubic centimeter. Boucher addresses this problem by using ultraviolet radiation in combination with compressional waves.
U.S. Pat. No. 3,212,756 to Hutton discloses a sound generator which can produce compressional waves at more than one frequency and acoustic pressure amplitude. Hutton recognizes that certain frequencies are more effective or necessary to kill particular types of bacteria.
Sonic and ultrasonic pasteurization techniques have not been widely adopted in industry because of several limitations which render them economically unattractive. One problem is the large amount of acoustical power necessary to bring about pasteurization. In large scale processes this high power requirement requires a very large investment in compressional wave resonators. Even with very large systems, the treatment time is substantial in order to sufficiently kill microorganisms. The high flow rates of large scale continuous flow processes further aggravates the acoustic power and treatment time problems because of the reduced time available for treating the liquid. High acoustic power requirements also require that more expensive types of acoustic resonators and transducers be used to achieve the necessary power density.
Some prior art pasteurizers have decreased the pasteurization time and power requirements by combining heating with compressional wave treatment. Although this may be acceptable in many situations, there are many processes where heating the liquid affects the taste or chemistry in detrimental ways. Examples of products adversely affected include milk, beer and soft drinks. The taste of beer in particular is characterized by the natural carbonization which occurs in the brewing process. Heat and compressional waves both detrimentally affect the taste of beer and accordingly there has for a substantial number of years been a need for a heat free pasteurization system which does not detrimentally affect the taste.
Prior art compressional wave pasteurization systems failed to properly pasteurize beer because in general they removed the natural carbonization. The Williams and Chambers patents discussed above addressed this problem with high static pressure to keep the gases in the beer. The prior art failed to recognize that beer could be regasified with the same carbonization gases which were removed when the beer was exposed to compressional waves. The current invention provides for regasification using the gases removed during compressional wave pasteurization treatment.
Another limitation of prior art acoustic pasteurizers was their failure to recognize that pasteurization involves two distinct stages. The first stage of pasteurization is degassing the liquid to remove substantially all dissolved gases. Degassing occurs whenever a liquid is subjected to compressional waves of sufficient acoustic pressure amplitude. The action is sometimes described as "gaseous cavitation" or in some cases just "cavitation". Degassing is needed because the presence of dissolved gases prevents effective vaporous cavitation from occurring until substantially all dissolved gases are removed. Gaseous cavitation must be distinguished from vaporous cavitation because it is during vaporous cavitation that microorganisms are effectively and quickly killed.
Failure of the prior art to recognize this important phenomenon has resulted in one step pasteurization processes which do not efficiently or quickly degas the liquid because the primary design consideration was producing cavitation rather than efficient degassing. The current invention has discovered that much greater efficiency can be obtained by using different frequencies of compressional waves and different compression wave patterns for degassing than for vaporous cavitation. This two stage treatment reduces the time necessary to degas and reduces the time required to kill bacteria, spores and other microorganisms using vaporous cavitation. Thus the overall treatment time can be greatly reduced.
Another limitation of the prior art pasteurization units was their failure to recognize that the acoustic power requirements necessary for pasteurization could be greatly reduced by carrying the process out under pressures reduced below atmospheric pressure. The acoustic power which must be generated by an acoustic transducer is a function of the square of the applied static pressure. Therefore it is possible to greatly reduce the required power by reducing the static pressure.
A further limitation of the prior art was the failure to recognize that degassing, could be greatly accelerated by varying or modulating the frequency of the compressional waves so that the frequency was maintained at or very near the resonant frequency of the bubbles produced during the process of degasification. Although it was known that compressional waves at or near the resonant frequency of a bubble lead to greater bubble growth rates. No one has heretofore recognized that varying the frequency to constantly coincide with the natural frequency of the bubble will greatly accelerate the growth rate. This invention also recognizes that power requirements can be reduced by modulating the instantaneous frequency of the compression waves to match the instantaneous frequency which is most effective at degassing or microorganism disintegration.
The converse of bubble growth is dissolving bubbles into a liquid or gasification. The concept of this invention to vary compressional wave frequency for maximizing the degassing rate also applies conversely to bubble diffusion and gasification so that greatly improved rates of gasification can be obtained using this method.
Other advantages and objectives of the invention will be apparent from the following detailed description.